One problem often encountered in the production of natural gas from underground reservoirs is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Natural gases which contain a significant amount of nitrogen may not be saleable, since they do not meet minimum heating value specifications and/or exceed maximum inert content requirements. As a result, the feed gas will generally undergo processing, wherein heavier components such as natural gas liquids are initially removed, and then the remaining stream containing primarily nitrogen and methane, and also possibly containing lower boiling or more volatile components such as helium, hydrogen and/or neon, is separated cryogenically. A common process for separation of nitrogen from natural gas employs a double column distillation cycle, similar to that used for fractionation of air into nitrogen and oxygen.
A recent significant advancement in such a process is described in Pahade et al. U.S. Pat. No. 4,878,932 wherein the feed is preseparated and the resulting liquid is partly vaporized to provide additional column vapor upflow resulting in improved methane recovery especially where the feed contains relatively low concentrations of nitrogen.
A problem with nitrogen-methane separation systems is the loss of some valuable methane with the nitrogen. This is especially the case where the feed additionally contains one or more lower boiling or more volatile components such as helium, hydrogen or neon and where recovery of such component(s) is desired. For example, when helium recovery is integrated into a cryogenic nitrogen-methane separation system, a portion of the nitrogen normally available as reflux is lost with the helium product. The reduction in the quantity and the quality of the nitrogen reflux results in an increased methane carryover thereby reducing the methane recovery.
Accordingly it is an object of this invention to provide an improved nitrogen-methane separation system.
It is another object of this invention to provide an improved nitrogen-methane separation system which can improve methane recovery where the feed additionally contains one or more lower boiling or more volatile components.